1. Field of the Invention
The present invention relates to a reciprocating linear actuator with high bearing stiffness.
2. Description of the Related Art
Die sets for molding diffraction gratings and light guide plates of liquid crystal display units are expected to be formed with tens of thousands of grooves with a pitch of several microns. Cutting these numerous grooves requires both high speed and high precision. Further, the die sets for diffraction gratings and light guide plates should be free from the smallest machining errors, with the result that smooth linear drive is essential in order to prevent vibration even during high-speed operation.
If a reciprocating motion is made at high speed in a conventional linear actuator, a reaction is caused by acceleration and deceleration. If machining requires a constant feed speed, in particular, the acceleration and deceleration are performed in a short stroke (i.e., in a short time), with the result that a greater reaction is produced. A precision processing machine that performs high-precision machining is supported by means of an air damper in order to suppress the propagation of floor vibration, so that it is easily shaken by a reaction of its drive shaft.
US Patent Application Publication No. 2007/010326 A1 (publication date: May 10, 2007), for example, discloses a method of canceling a reaction that is produced when a slide of a reciprocating linear actuator is reciprocated at high speed with respect to a guide. In the reciprocating linear actuator disclosed in this patent document, the guide and the slide that are movable coaxially with a base as a fixed portion are supported on the base through a fluid dynamic bearing, and the guide receives a reaction of acceleration and deceleration of the slide, so that the reaction cannot be transmitted to the outside.
FIGS. 9 and 10 are views illustrating the reciprocating linear actuator disclosed in the patent document mentioned above. FIG. 9 is an external perspective view of the reciprocating linear actuator, and FIG. 10 is a sectional view of the actuator taken along line X-X of FIG. 9.
A base 50 of the reciprocating linear actuator is fixedly set in a predetermined position in a machine tool (not shown). The base 50 supports the opposite end portions of a guide 51 with the aid of first bearings A. A slide 52 is supported on the guide 51 for movement in the axial direction of the guide by a second bearing B. The slide 52 and the guide 51 are movable in the same direction. By moving in opposite directions, the guide 51 and the slide 52 can cancel a reaction that is produced when the reciprocating linear actuator is driven. Permanent magnets 53 for reversal are mounted on the guide 51. Further, a coil and permanent magnets for reversal (not shown) that constitute a linear motor are mounted within the slide 52. Fluid bearings can be used for the first and second bearings A and B.
In the reciprocating linear actuator disclosed in the patent document mentioned above, if an external force in any other direction than a driving direction acts on the slide 52, it is transmitted to the slide 52, the bearing B between the guide 51 and the slide 52, the guide 51, the bearing A between the base 50 and the guide 51, and the base 50, in the order named. In short, the external force applied to the slide 52 is transmitted to the base 50 through the first and second bearings A and B.
A fluid bearing (air bearing), in particular, is a part that is lower in stiffness than any other structural parts, and it can be regarded as an elastic body that is displaced in proportion to an applied force. If the force is transmitted through the two bearings A and B, it practically means that two elastic bodies are connected in series, so that the stiffness of the bearings is reduced.
When the reciprocating linear actuator is driven, moreover, the guide 51 and the slide 52 move simultaneously, so that the straight motion accuracy of the slide 52 with respect to the base 50 is determined by combining the straight motion accuracy of the guide 51 with respect to the base 50 and the straight motion accuracy of the slide 52 with respect to the guide 51. Thus, such combined straight motion accuracy tends to be poorer than a straight motion accuracy resulting from a simple structure composed of only the guide 51 and the slide 52.
As shown in FIG. 10, the straight motion accuracy of the slide 52 is influenced by only the component shape accuracy of the guide 51. In reality, however, the straight motion accuracy of the slide 52 is influenced not only by the straightness of that part (bearing B) of the guide 51 on which the slide 52 moves but also by the straightness of that part (bearing A) of the guide 51 which moves relatively to the base 50, as mentioned before. Thus, the overall straightness of the guide 51 influences the straight motion accuracy of the slide 52, with the result that the guide 51 must be finished with high precision throughout the length of the guide 51.
As mentioned before, the conventional reciprocating linear actuator has a problem that the bearing stiffness of the slide 52 with respect to the base 50 is reduced due to the use of the double bearing structure, including the bearing A between the base 50 and the guide 51 and the bearing B between the slide 52 and the guide 51. There is another problem that the straight motion accuracy of the slide 52 during the drive is reduced as the resultant of the straight motion accuracy of the slide 52 with respect to the guide 51 and that of the guide 51 with respect to the base 50.